An optical fiber is used for optical transmission, illumination, decoration, a display, and the like. An inorganic glass-based optical fiber exhibits excellent optical transmission property over a wide range of wavelengths, but there are problems that the processability thereof is poor and the mechanical durability thereof is weak. On the other hand, a plastic optical fiber (hereinafter, it is referred to as “POF”) has a structure in which a core is mainly formed of a resin with high transparency, such as polymethyl methacrylate, and the periphery thereof is cladded with a transparent resin with a lower refractive index than that of the core. Hence a POF is characterized in that the flexibility and the processability thereof are superior to a glass-based optical fiber. In addition, the transmittable distance of a POF has been increased as the production technique thereof has been improved, and thus a POF is used as a transmission medium of optical information for shorter distance LAN or various communications.
In general, at the time of using a POF, a POF is rarely used as a single body of POF except for illumination application, but is often used as a POF cable in which a POF is covered with a thermoplastic resin in order to impart mechanical strength, heat resistance, flame retardance or the like. Particularly in recent years, the regulation on flame retardance of a plastic product has become strict, and a POF cable with high flame retardance is also desired.
A POF itself is formed of an inflammable plastic such as polymethyl methacrylate, and thus is easily burned. For that reason, it is possible to impart flame retarding effect to a POF to a certain extent by forming into a POF cable by covering a POF with a resin having high flame retardance, for example, a vinyl chloride resin or a chlorinated polyethylene resin, or a flame retardant resin dispersed with a large amount of various kinds of flame retardants such as a halogen-based one, a phosphorus-based one, magnesium hydroxide, and the like.
For example, in Patent Literature 1, a flame retarding POF code, in which the periphery of a POF is covered with a polyethylene-based resin composition including magnesium hydroxide and red phosphorus, is suggested.
In Patent Literature 2, a POF cable, in which the outside of a POF is covered with a first jacketing layer of a resin that is not heat fused with a cladding and the outside of the first jacketing layer is covered with a fluorine-based resin, is suggested.
In addition, in Patent Literature 3, there is a suggestion on a POF cable, in which the outside of a POF is covered with a polyolefin resin including magnesium hydroxide and the outside of the polyolefin resin is covered with a resin including a vinylidene fluoride-chlorotrifluoroethylene-based copolymer.